var banner = new Array();

banner[0]='http://img808.imageshack.us/img808/2742/38775738.png';
banner[1]='http://img51.imageshack.us/img51/1508/78229617.png';
banner[2]='http://img607.imageshack.us/img607/9053/44783938.png';

document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];

setInterval("document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];",9000);